An exercise in constructing multi-phase communication protocols

Many real-life protocols can be observed to go through different phases performing a distinct function in each phase. We present a multi-phase model for such protocols. A phase is formally defined to be a network of communicating finite state machines with certain desirable correctness properties; these include proper termination, and freedom from deadlocks and unspecified receptions. A multi-function protocol is constructed by first constructing separate phases to perform its different functions. We discuss how to connect these phases together to implement the multi-function protocol such that the resulting network of communicating finite state machines is also a phase (i.e. it possesses the desirable properties defined for phases). A high-level session control protocol modeled after one in IBM's Systems Network Architecture is discussed, and constructed as a multi-phase protocol.